| 1. |
- Puttnam, Benjamin J., et al.
(författare)
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Characteristics of homogeneous multi-core fibers for SDM transmission
- 2019
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Ingår i: APL Photonics. - : AIP Publishing. - 2378-0967. ; 4:2
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Tidskriftsartikel (refereegranskat)abstract
- We describe optical data transmission systems using homogeneous, single-mode, multi-core fibers (MCFs). We first briefly discuss space-division multiplexing (SDM) fibers, observing that no individual SDM fiber offers overwhelming advantages over bundles of single-mode fiber (SMF) across all transmission regimes. We note that for early adoption of SDM fibers, uncoupled or weakly coupled fibers which are compatible with existing SDM infrastructure have a practical advantage. Yet, to be more attractive than parallel SMF, it is also necessary to demonstrate benefits beyond improved spatial spectral efficiency. It is hoped that the lower spread of propagation delays (skew) between spatial channels in some fibers can be exploited for improved performance and greater efficiency from hardware sharing and joint processing. However, whether these benefits can be practically harnessed and outweigh impairments or effort to mitigate cross talk between spatial channels is not yet clear. Hence, focusing on homogeneous MCFs, we first describe measurements and simulations on the impact of inter-core cross talk in such fibers before reporting experimental investigation into the spatial channel skew variation with a series of the experimental results including a comparison with SMF in varying environmental conditions. Finally, we present some system and transmission experiments using parallel recirculating loops that enable demonstration of both multi-dimensional modulation and joint digital processing techniques across three MCF cores. Both techniques lead to increased transmission reach but highlight the need for further experimental analysis to properly characterize the potential benefits of correlated propagation delays in such fibers.
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| 2. |
- Alfredsson, Arni, 1989, et al.
(författare)
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Joint Phase Tracking for Multicore Transmission with Correlated Phase Noise
- 2018
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Ingår i: - 9781538653432 ; , s. 16-17
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Konferensbidrag (refereegranskat)abstract
- Space-division multiplexed transmission over multicore fibers offers potential for joint-core processing to compensate for correlated phase noise. We review methods that take advantage of the phase-noise correlation across cores and assess their benefits in terms of transmission reach and pilot-rate requirements.
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| 3. |
- Alfredsson, Arni, 1989, et al.
(författare)
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Pilot-Aided Joint-Channel Carrier-Phase Estimation in Space-Division Multiplexed Multicore Fiber Transmission
- 2019
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Ingår i: Journal of Lightwave Technology. - 0733-8724 .- 1558-2213. ; 37:4, s. 1133-1142
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Tidskriftsartikel (refereegranskat)abstract
- The performance of pilot-aided joint-channel carrier-phase estimation (CPE) in space-division multiplexed multicore fiber (MCF) transmission with correlated phase noise is studied. To that end, a system model describing uncoded MCF transmission where the phase noise comprises a common laser phase noise, in addition to core- and polarization-specific phase drifts, is introduced. It is then shown that the system model can be regarded as a special case of a multidimensional random-walk phase-noise model. A pilot-aided CPE algorithm developed for this model is used to evaluate two strategies, namely joint-channel and per-channel CPE. To quantify the performance differences between the two strategies, their respective phase-noise tolerances are assessed through Monte Carlo simulations of uncoded transmission for different modulation formats, pilot overheads, laser linewidths, numbers of spatial channels, and degrees of phase-noise correlation across the channels. For 20 GBd transmission with 200 kHz combined laser linewidth and 1% pilot overhead, joint-channel CPE yields up to 3.4 dB improvement in power efficiency or 25.5% increased information rate. Moreover, through MCF transmission experiments, the system model is validated and the strategies are compared in terms of bit-error-rate performance versus transmission distance for uncoded transmission of different modulation formats. Up to 21% increase in transmission reach is observed for 1% pilot overhead through the use of joint-channel CPE.
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| 4. |
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| 5. |
- Alfredsson, Bo, et al.
(författare)
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Low temperature creep in a high strength roller bearing steel
- 2016
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Ingår i: Mechanics of materials. - : Elsevier. - 0167-6636 .- 1872-7743. ; 100, s. 109-125
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Tidskriftsartikel (refereegranskat)abstract
- Noticeable low temperature creep was established for a bainitic and a martensitic microstructure of the 100CrMnMo8 high strength roller bearing steel. The response revealed primary creep that differed between the microstructures, following a power law for martensite and the logarithmic description for bainite. The detected creep was pressure sensitive, higher in tension than in compression for the same stress level, following the strength differential effect (SDE) at material yielding. Two models were proposed where the stress variable for the pressure effect was based on the Drucker-Prager yield function and deviatoric creep strains were derived from a non-associated von Mises potential. Model parameters were determined from experimental series on the respective microstructure. When the models were evaluated against the experiments the accuracy was of the same order as the effects of different heat treatment batches and different load application rates. The importance of different material parameters in the descriptions was discussed.
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| 6. |
- Appelquist, Elinor, et al.
(författare)
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Investigation of the Global Instability of the Rotating-disk Boundary Layer
- 2015
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Ingår i: Procedia IUTAM. - : Elsevier. - 2210-9838. ; , s. 321-328
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Konferensbidrag (refereegranskat)abstract
- The development of the flow over a rotating disk is investigated by direct numerical simulations using both the linearized and fully nonlinear incompressible Navier-Stokes equations. These simulations allow investigation of the transition to turbulence of the realistic spatially-developing boundary layer. The current research aims to elucidate further the global linear stability properties of the flow, and relate these to local analysis and discussions in literature. An investigation of the nonlinear upstream (inward) influence is conducted by simulating a small azimuthal section of the disk (1/68). The simulations are initially perturbed by an impulse disturbance where, after the initial transient behaviour, both the linear and nonlinear simulations show a temporally growing upstream mode. This upstream global mode originates in the linear case close to the end of the domain, excited by an absolute instability at this downstream position. In the nonlinear case, it instead originates where the linear region ends and nonlinear harmonics enter the flow field, also where an absolute instability can be found. This upstream global mode can be shown to match a theoretical mode from local linear theory involved in the absolute instability at either the end of the domain (linear case) or where nonlinear harmonics enter the field (nonlinear case). The linear simulation grows continuously in time whereas the nonlinear simulation saturates and the transition to turbulence moves slowly upstream towards smaller radial positions asymptotically approaching a global upstream mode with zero temporal growth rate, which is estimated at a nondimensional radius of 582.
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| 7. |
- Appelquist, Ellinor, et al.
(författare)
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Transition to turbulence in the rotating-disk boundary-layer flow with stationary vortices
- 2017
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 836, s. 43-71
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Tidskriftsartikel (refereegranskat)abstract
- This paper proposes a resolution to the conundrum of the roles of convective and absolute instability in transition of the rotating-disk boundary layer. It also draws some comparison with swept-wing flows. Direct numerical simulations based on the incompressible Navier-Stokes equations of the flow over the surface of a rotating disk with modelled roughness elements are presented. The rotating-disk flow has been of particular interest for stability and transition research since the work by Lingwood (J.FluidMech., vol.299, 1995, pp.17-33) where an absolute instability was found. Here stationary disturbances develop from roughness elements on the disk and are followed from the linear stage, growing to saturation and finally transitioning to turbulence. Several simulations are presented with varying disturbance amplitudes. The lowest amplitude corresponds approximately to the experiment by Imayama etal. (J.FluidMech., vol.745, 2014a, pp.132-163). For all cases, the primary instability was found to be convectively unstable, and secondary modes were found to be triggered spontaneously while the flow was developing. The secondary modes further stayed within the domain, and an explanation for this is a proposed globally unstable secondary instability. For the low-amplitude roughness cases, the disturbances propagate beyond the threshold for secondary global instability before becoming turbulent, and for the high-amplitude roughness cases the transition scenario gives a turbulent flow directly at the critical Reynolds number for the secondary global instability. These results correspond to the theory of Pier (J.EngngMaths, vol.57, 2007, pp.237-251) predicting a secondary absolute instability. In our simulations, high temporal frequencies were found to grow with a large amplification rate where the secondary global instability occurred. For smaller radial positions, low-frequency secondary instabilities were observed, tripped by the global instability.
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| 8. |
- Medici, D., et al.
(författare)
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The upstream flow of a wind turbine : blockage effect
- 2011
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Ingår i: Wind Energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 14:5, s. 691-697
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Tidskriftsartikel (refereegranskat)abstract
- The flow upstream a wind turbine is studied in order to investigate blockage effects. We use rotating wind turbine models in a wind tunnel, where velocity measurements have been made both with hot-wire anemometry up to approximately 4.5 diameters (D) upstream the turbine, as well as laser particle image velocimetry measurements close to the turbine rotor. Also, numerical simulations have been carried out by means of a finite volume code. The measurements show, among other things, that the flow is affected more than 3 D upstream the rotor plane.
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| 9. |
- Segalini, Antonio, et al.
(författare)
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Gust structure and generation in canopy flows
- 2011
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Ingår i: European Wind Energy Conference and Exhibition 2011. - 9781618399915 ; , s. 217-221
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Konferensbidrag (refereegranskat)abstract
- Due to an increased demand to place wind turbines in forest areas it is of importance to know the details of the wind flow field over forest canopies. In this study we compare the flow characteristics of an atmospheric boundary layer over a forest canopy and a wind tunnel canopy model. Both the mean velocity distribution and the turbulence intensities were measured for both cases and the friction velocity was determined from the Reynolds shear stress above the edge of the canopy. The comparison between the two data sets is reasonably good, especially close to the canopy edge. The analysis of the time series of both measurement sets has highlighted that the turbulent canopy flow (in both wind tunnel and atmospheric data sets) is composed, beside the high frequency turbulence, by some flow structures detectable only through phase averages approaches.
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| 10. |
- Örlü, Ramis, 1977-, et al.
(författare)
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Generalized diagnostic scaling for high-order moments in turbulent boundary layers
- 2015
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Ingår i: Proceedings - 15th European Turbulence Conference, ETC 2015. - : TU Delft.
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Konferensbidrag (refereegranskat)abstract
- The present work builds upon the diagnostic plot for the streamwise turbulence intensity [Alfredsson & Örlü, 2010] and generalises it for higher-order (even and odd) moments providing a general description of the probability density distribution of streamwise velocity fluctuations. Turbulent boundary layers (up to a friction Reynolds number of 20’000) are employed and demonstrate its feasibility to scale data throughout the overlap and outer region.
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